In a conventional type of voltage translator, a pair of bipolar transistors are connected in a differential configuration for converting an input signal into an output signal at a different voltage level. FIG. 1 illustrates such a prior art device as employed in the input section of the 8T14 integrated circuit manufactured by Signetics Corporation. In this translator, an input voltage V.sub.IN is applied to the base of an NPN transistor Q1 whose emitter is connected through a node N1 to the emitter of an identical NPN transistor Q2. An output voltage V.sub.OUT is taken between the collector of transistor Q2 and a 1,680-ohm resistor R1 connected to a source of a supply voltage V.sub.CC at 5 volts. A PN junction diode J1 between the collector of transistor Q1 and the V.sub.CC supply protects it from receiving current from the input. A reference voltage V.sub.REF1 is supplied to the base of transistor Q2 from an NPN transistor Q3 which forms part of a voltage divider further consisting of resistors R2, R3, and R4 and a PN junction diode J2 connected as shown between the V.sub.CC supply and a source of another supply voltage V.sub.EE at ground reference (0 volt). A hysteresis input V.sub.HYS supplied to the base of transistor Q3 causes voltage V.sub.REF1 to vary about .+-.0.2 volt from its nominal 1.5-volt operating point.
A substantially constant current is provided to the emitters of transistors Q1 and Q2 from a current source consisting of an NPN transistor Q4 which is part of a current mirror that also includes NPN transistors Q5 and Q6, a 1,400-ohm resistor R5, and a 4,000-ohm resistor R6 connected as shown. Resistor R5 fixes the current through transistor Q5. This current is then mirrored in transistor Q4. More particularly, the current mirror establishes a reference voltage V.sub.REF2 of about 1 V.sub.BE at the base of transistor Q4 to make it conductive. V.sub.BE is the standard base-to-emitter voltage for an NPN transistor when its base-emitter junction is just conductively forward biased. Resistors R7 and R8 in the emitters of transistors Q4 and Q5, respectively, are small 50-ohm resistors that act to make the current source less sensitive to noise and substantially do not effect voltage V.sub.REF2. Such resistors are often not included in a current mirror.
In operation, transistor Q1 turns on when voltage V.sub.IN is raised to a logical high value above V.sub.REF1. This causes transistor Q2 to turn off, and voltage V.sub.OUT goes to a logical high value near 5 volts. When voltage V.sub.IN drops to a logical low value below V.sub.REF1, transistor Q1 turns off and transistor Q2 turns on to bring voltage V.sub.OUT down to a logical low value around 0.8 volt.
While the input impedance of this translator is high, its resistors are not directly ratioed for V.sub.BE tracking. A separate current mirror must be provided for each such translator used in an integrated circuit. In addition, a V.sub.IN excursion above 5-6 volts could degrade the translator and allow current to flow into the V.sub.CC supply by causing the emitter-base junction of transistor Q2 to break down.